Having high transparency, good mechanical characteristics, and good electric characteristics, biaxially orientated polypropylene film have been applied in various fields including packaging materials, tape materials, and electric materials such as cable wrapping and capacitors.
In the field of capacitor production, in particular, such film has been preferred for high voltage capacitors, for both direct current and alternating current because of its good withstand voltage characteristics and low loss characteristics.
In recent years, increasing numbers of electric devices have been replaced with inverter-based ones and, accordingly, there are stronger demands for smaller-sized, large-capacity capacitors. To meet demands in such markets, particularly for automobiles (including hybrid cars), photovoltaic power generators and wind power generators, there is an urgent demand for biaxially orientated polypropylene film products that have improved voltage withstanding characteristics and further reduced thicknesses while maintaining high productivity and processability.
Biaxially orientated polypropylene film used in high voltage capacitors should have moderately roughed surfaces to maintain voltage withstanding characteristics, productivity, and processability. Such roughness is important to improve slip properties and oil impregnation properties and impart security protection properties particularly in vapor deposited capacitors. Security protection properties of a metal-deposited capacitor composed of dielectric film and metal-deposited film formed thereon to work as electrode are intended to prevent short-circuiting in the event of abnormal electric discharge by applying electric discharge energy so that the deposited metal is broken to recover insulating properties, which is a very useful function to ensure safety.
Methods proposed so far to roughen the surface of biaxially orientated polypropylene film include mechanical methods such as embossing and sand blasting, chemical methods such as chemical etching with a solvent, stretching of a sheet containing a dissimilar polymer such as polyethylene, and stretching of a sheet containing β crystals (for example, see Japanese Patent Application Laid-open Nos. 51-63500 and 2001-324607).
However, mechanical and chemical methods cannot achieve a sufficiently high roughness density and stretching a β crystal-containing sheet under common conditions tends to form bulky protrusions, possibly failing to produce a surface that is acceptable in terms of roughness density, bulky protrusions and number of protrusions. When using film having a surface roughened by these methods, furthermore, oil impregnation between film layers will not occur to a sufficient degree during a capacitor production process and, accordingly, such film layers tend to contain partially unimpregnated portions, possibly leading to a short capacitor life. The method of stretching a sheet containing a dissimilar polymer such as polyethylene does not suffer from significant residues of bubbles during formation of a capacitor, but such a dissimilar polymer can have an adverse influence during recycling of the film, possibly leading to low recyclability.
When any of the methods is used, biaxially orientated polypropylene film with a roughened surface may not have sufficient security protection properties under severe use conditions where the voltage gradient is 370 V/μm or more at high temperatures, leading to problems in terms of reliability. The voltage gradient refers to the voltage applied per unit thickness of the film.
To improve roughness density and ensure uniformity of protrusions, there are some proposals including polypropylene film having a high melt tension (see, for example, Japanese Patent Application Laid-open No. 2001-72778) and layered film containing such polypropylene film having a high melt tension stacked on common polypropylene film (see, for example, Japanese Patent Application Laid-open No. 2001-129944). If the polypropylene resin having a high melt tension is used in capacitors, it may be impossible to achieve a sufficient degree of heat resistance and withstand voltage characteristics, leading to the problem of a considerable reduction in dielectric breakdown voltage, particularly at high temperatures.
In the case of the technique of producing layered film containing polypropylene resin having a high melt tension, it is actually impossible to produce a uniform layered structure, particularly when thin film layers with a thickness of 5 μm or less are used, failing to provide dielectric film that has satisfactory practical quality.
In addition, Japanese Patent No. 3508515 proposes biaxially orientated polypropylene film having a surface with a controlled degree of roughening and a production method thereof. However, the technique proposed in JP '515 does not work sufficiently and has difficulty producing a roughed film surface with a low protrusion level.
For JP '515 and Japanese Patent Application Laid-open No. 2007-308604, furthermore, it is insisted that both good element winding properties and voltage withstanding characteristics can be realized simultaneously if an unstretched sheet with a β crystal fraction in a specific range is used to form a film in which at least one surface is a finely roughened one. The production methods disclosed in JP '515 and JP '604, however, do not work sufficiently in controlling the roughness of both surfaces of a film and, in particular, do not provide film that has a sufficiently high surface roughness to achieve a high withstand voltage, productivity, and processability required for automobile members.
There are some disclosed techniques (see, for example, Japanese Patent Application Laid-open No. 2011-122143) designed to control the roughness of both surfaces of a film in a predetermined range by adjusting the air temperature of the air knife, which works to bring an unstretched sheet in strong contact with the casting drum, to 60° C. to 120° C. during the melt-extrusion of resin material for biaxially orientated polypropylene film. However, when applied to uses that require a high withstanding voltage to allow the capacitor to have a voltage gradient of 450 V/μm or more at high temperatures, the technique disclosed in JP '143 may fail to realize a required level of security protection properties, possibly leading to problems in terms of reliability.
It could therefore be helpful to provide a biaxially orientated polypropylene film for capacitors that ensures a high withstanding voltage and reliability as well as stable productivity and element processability when applied to high voltage capacitors.
Such biaxially orientated polypropylene film for capacitors provides a biaxially orientated polypropylene film in which both surfaces are smooth and contain highly uniform protrusions suitable for production of capacitors and the like, a high roughness density, and few bulky protrusions.